Escape to form a union (5)

I believe the answer is:

elope

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'escape' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for elope that I've seen before include "Run away in secret" , "(Lovers) run away" , "Run away together" , "Do a runner" , "go off to the match?" .)

I've seen this clue in The Mirror.

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